This invention relates to a video switcher with independent processing of selected video signals.
FIG. 1 illustrates a conventional video mixing system, or switcher, in simplified block form. The FIG. 1 switcher comprises a source selector 2 which receives multiple video input signals, e.g. from external video signal sources, and provides selected ones of these input signals to mix/effect amplifiers (M/Es) 4. Each M/E receives at least two video input signals and combines them under control of signals provides by an operator interface 6. A typical M/E that receives four video input signals is shown in FIG. 2. FIG. 2 illustrates an M/E in functional form, and does not necessarily represent its physical arrangement.
The four video input signals received by the FIG. 2 M/E are a title signal, a foreground signal and two background signals. The title video signal is associated with a title key which defines the area of the picture field that is occupied by a title image, and similarly the foreground video signal has a foreground key which defines the area of the picture field occupied by a foreground image. The two background signals are generally full field video signals and do not have keys associated there with.
It will be appreciated that the paths from the sources that provide signals having keys associated therewith must be dual paths, one path for the video signal and one for the key.
The four video signals received by the M/E shown in FIG. 2 are combined in a background mixer 42, a video key mixer 43, and a title key mixer 44 under control of signals provided by the operator interface through a background mixer control 45, a video key mixer control 46, and a title key mixer control 47. The output video signal represents a picture consisting of a title, a foreground image and a background image. The background image may be represented by a selected one of the background signals or by a mix of the two background signals. The title appears to be in front of the foreground, and the foreground appears to be in front of the background.
In a conventional switcher, an output video signal having contributions from a large number of input video signals (many more than four) is created by use of multiple M/Es. Each M/E 4 is capable of receiving several video signals and combining them, and the output signal provided by the M/E can be re-entered into the source selector as shown at 8 so that it serves as an input for another M/E. In FIG. 1, each M/E 4 is shown as having re-entry connections to each other M/E 4 and to a program/preset M/E 10.
A video signal representing a complex scene may be produced by recording a video signal, playing it back and modifying it in some manner using an M/E, and re-recording the modified signal. This sequence of playing back, modifying and re-recording the video signal is repeated until the desired composite picture is obtained. The composite picture may be regarded as being composed of multiple layers, some partially concealing others and being concealed by yet others, with the positions of the boundaries between the visible portions of the various layers being variable under operator control. In producing a signal representing a picture composed of multiple layers, it is desirable that several layers be added in a single playback and re-record pass to the picture represented by the signal that was previously recorded, instead of adding only one layer on each pass.
Conventional M/Es operate by performing one or more two-channel mixes. Each mixer receives two video signals, e.g. the video signal that was recorded on the previous pass and is being played back and an external video signal that is to be combined with the previously-recorded signal, and acts on them in a complementary fashion in dependence on a mix control signal. For example, as the amplitude of the contribution of one input video signal (video A) to the output signal is increased, the amplitude of the contribution of the other input video signal (video B) is necessarily decreased. Accordingly, the operator of the switcher is not able to consider the effect of an action on one input video signal in isolation, and must also consider the effect that the same action is having on the other input video signal. If the operator is attempting to manipulate multiple layers, the interaction between layers makes it difficult to ensure that the desired effect is obtained.